1. Field of the Invention
The present invention relates to an electronic endoscope device for obtaining images with an image pick-up element which captures an object image by accumulating an electric charge, and to a signal processing unit used in this electronic endoscope device.
2. Related Art Statement
Electronic endoscope devices have been widely used for observing the digestion tract, for example, esophagus, stomach, small intestines, and large intestines, or bronchial tube such as lungs by inserting an endoscope into a body cavity, and, if necessary, conducting a variety of medical treatments by using therapeutic instruments inserted into the dedicated channels.
Electronic endoscope devices of a field sequential system are known as electronic endoscopes in which a rotary optical filter is provided in a light source unit, the object is sequentially illuminated with red, green, and blue light from this light source unit, the light thus obtained from the object is received by monochromatic image pick-up elements, signal processing is conducted in a signal processing unit, and the color image obtained is outputted to a display unit. Because of differences in wavelength characteristics of optical filters or spectral sensitivity characteristics of the image pick-up elements among the electronic endoscope devices of a field sequential system, adjustment of white balance has been conducted by using electronic circuits in typical electronic endoscope devices.
The white balance adjustment is conducted by pushing down a white-balance setting switch in a state that the image of a white body has been picked up. When the switch is pushed down, the white balance adjustment circuit adjusts the color balance such that the ratio of the amplitudes of the B signal and R signal to the G signal comes to a prescribed value. In case of analog circuitry, this adjustment typically involves comparing the outputs of all the signals by a comparator and adjusting little by little the amplification factors so as to obtain convergence to the prescribed color. However, in this case, a large number of frame images are required before the correct white color adjustment is completed and, therefore, the adjustment is a time-consuming procedure.
On the other hand, the adjustment of color balance with digital circuitry is most often conducted by sampling signals of each color within one frame, directly computing the amplification factors of B signal or R signal with a CPU or the like from the intensity ratio thereof, and amplifying each signal by using a digital multiplier. In this case, the adjustment of white balance does not take a long time.
Further, diagnostics employing electronic endoscope devices has recently begun involving auto-fluorescence observations using auto-fluorescence of living-body tissue, in addition to normal observations in which color images similar to those observed by the naked eye are displayed on a monitor.
In the auto-fluorescence observations, diagnostics is conducted according to the fact that spectra of auto-fluorescence emitted from the living-body tissue when it is illuminated with excitation light within a UV to blue range is different between the normal mucous membrane and a tumor. In this case, a lesion portion can be clearly recognized by displaying the image of auto-fluorescence into the reflected light image returned due to reflection from the living-body tissue on a display, the reflected light image having allocation of respective different colors. At this time, the wavelength of the light illuminated is restricted by using narrow-band optical filters (see, for example, Japanese Patent Application Laid-open No. 2002-95635) such that the spectral distribution in each wavelength band becomes discreet.
Furthermore, in the conventional electronic endoscope devices, the adjustment of color balance is typically conducted by employing a white object as a reference (see, for example, Japanese Patent Application Laid-open No. 2002-336196), but there are also electronic endoscope devices in which lesion portions in any patient can be observed with a fixed color tone by adjusting the color balance by employing the color of the normal mucous membrane of the patient as a reference during auto-fluorescence observations.
In such conventional electronic endoscope devices, the amplification factor of the amplifier of the signal processing unit has been adjusted for each color (each wavelength band) to adjust the color balance. For this reason, a small difference in the amplification factors between the colors has not caused significant problems, but when the difference in the amplification factors between the colors is large, noise is often increased by a color component of the image with a high amplification factor.
In particular, in electronic endoscope devices for fluorescent observations, the difference in brightness of the fluorescence emitted by mucous membranes of the patients is large among the patients, and the difference in amplification factors between the colors has to be increased to correct the aforementioned difference in brightness which could result in images with a high noise level.
Furthermore, because fluorescence is extremely weak, the quantity of illumination light for obtaining a reflected light image has to be made lower than the quantity of illumination of excitation light. For this purpose, the transmission wavelength band of the filter is narrowed or transmissivity thereof is decreased. This could easily lead to a wavelength error during optical filter manufacture and produce a relatively large negative effect on the image color. For example, in case of a 10 nm error in a filter with a transmission band with a half-width of 100 nm, the error of the transmitted light intensity is about 10%, but in case of a 10 nm error in a filter with a transmission band with a half-width of 20 nm, the error of the transmitted light intensity is about 50%. As a result, the difference in amplification factors between the colors easily became large during color balance adjustment and the probability of obtaining images with a high level of noise is large.
Furthermore, in case of endoscope devices equipped with electronic shutters, color correction can be also conducted by controlling the exposure period. However, in endoscope devices requiring light shielding with a rotary filter plate or the like, the exposure period that is electrically set and the exposure quantity falling upon the image pick-up element do not show a simple linear relationship. This is because the luminous flux crossing the rotary filter is not perfectly converged in one spot but has a certain surface area.
FIG. 11 is a graph illustrating the relationship between the exposure quantity (in case no electronic shutter is used) and time with respect to such a rotary filter. FIG. 12 is a graph illustrating the relationship between the exposure quantity and exposure period with respect to the rotary filter.
The ideal relationship between the exposure quantity and time obtained when one filter in the rotary filter is inserted in the luminous flux is represented by a broken line 91 shown in FIG. 11, but actual relationship is represented by a solid line 92 shown in FIG. 11.
Furthermore, referring to FIG. 11, if an assumption is made that an electronic shutter is used and the electric charge sweep is carried out at the point A of time and that the exposure period is ended at the point B of time, then the exposure period will be the A-B period in FIG. 11. The relationship between the exposure period and exposure quantity at this time is shown in FIG. 12. Referring to FIG. 12, ideally speaking, the exposure period and exposure quantity are preferably proportional to each other, as shown by a broken line 93, but in reality they show a complex relationship represented by a solid line 94. Furthermore, the characteristics of the solid line 92 shown in FIG. 11 or solid line 94 shown in FIG. 12 differ depending on the type or individual features of the endoscope or light source unit. For this reason, in the electronic endoscope devices using rotary filter, a strict color balance has been difficult to set only by simple adjustment of exposure period.